1. Field of the Invention
This invention relates to a method and apparatus for monitoring the approach to criticality of the core of a light water moderated nuclear reactor which is provided with a localized artificial neutron source.
2. Description of the Prior Art
In general terms, a nuclear reactor contains a mass of fissionable material arranged in the reactor core to produce controlled fission reactions. The fission reactions occur when free neutrons at the proper energy level strike the atoms of the fissionable material resulting in the release of large amounts of heat energy which are extracted from the core in the reactor coolant and in the release of additional free neutrons which are available to produce more fission reactions. Some of these released neutrons escape or are absorbed by neutron absorbers within the core and therefore do not cause additional fission reactions. By controlling the amount of neutron absorbant material present in the core, the rate of fission can be controlled. There are always random fission reactions occurring in the fissionable material but when the core is shutdown the released neutrons are absorbed at such a high rate that a sustained series of reactions does not occur. By reducing the neutron absorbant material until the number of neutrons in a given generation equals the number of neutrons in the previous generation, the process becomes a self-sustaining chain reaction and the reactor is said to be critical. When the reactor is critical, the neutron flux is six or so orders of magnitude higher than when the core is shutdown. In order to accelerate the increase in the neutron flux in the shutdown core to achieve practical transition intervals, an artificial neutron source is implanted in the reactor core among the fuel cells containing the fissionable material. This artificial neutron source creates a localized increase in the neutron flux to aid in bringing the reactor up to power.
In the absence of a neutron source, the ratio of the number of free neutrons in one generation to those in the previous generation is referred to as the multiplication factor, K, and is used as a measure of the reactivity of the reactor core. Thus when the reactor is critical, K is equal to one and K remains equal to one over the full power range of the reactor. The increase in the neutron population as the reactor goes from subcritical to critical is not a linear function but rises approximately exponentially as a K 1 is approached. Thus, for practical reasons, a K of 0.99 been recognized as significant, and in fact regulations stipulate that certain actions can be taken on a shutdown reactor only when the multiplication factor is below this value.
It is therefore evident that it is very desirable to have a means for determining when the multiplication factor of a nuclear reactor is above and when it is below 0.99. However, presently it is difficult to accurately determine the K value on an on-line basis. Under current practice, the reactivity of a shutdown reactor is determined as a function of the inverse count rate ratio. The count rate is a measure of the reactor neutron flux expressed as a function of the number of neutrons detected by a neutron detector in a unit time period. The count rate for the latest unit time period is divided into the count rate for a reference time period to generate the inverse count rate ratio. The inverse ratio is used since it will approach zero as the reactor approaches criticality. If the direct count rate ratio were used instead, the ratio would get very large as criticality was approached, but the large number is relative and does not provide a meaningful statement of the reactivity of the core. Even the inverse count rate ratio is not very accurate since it depends upon the reference time period selected. While an operator can gain a feel for the approach to criticality by observing the rate at which the inverse count rate approaches zero, even this technique loses its value if the increase in reactivity is levelled off for any reason subsequent to the reference time period and then resumed. As a result, the regulators and operators tend to be very conservative with regard to the approach to criticality since with the current practice there is no accurate on-line system for determining the shutdown margin of the reactor when the multiplication factor is in the neighborhood of 0.99.
Accordingly, it is the primary object of this invention to generate an accurate on-line indication of the reactivity of a nuclear reactor as criticality is approached.